<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="../js/vue.js"></script>
</head>
<body>
    <div id="root">
       姓 <input v-model="firstname"/>
       名<input v-model="lastname">
       <div>{{fullname}}</div>
            </div>
        <script>
            new Vue({
                el:"#root",
                data:{
                    firstname:'',
                    lastname:'',
                    count:0
                },
                computed:{
                    fullname:function(){
                        return this.firstname+' '+this.lastname
                    }
                },
                watch:{
                    fullname:function(){
                        this.count++
                    } ,
                    lastname:function(){
                        this.count++
                    }
                }               
            })
        </script>

</body>
</html>